Polymer electrolyte membrane or proton exchange membrane (PEM) fuel cells have intrinsic benefits and a wide range of applications due to their relatively low operating temperatures (room temperature to approximately 80° C.). The active portion of a PEM is a membrane sandwiched between an anode and a cathode layer. Fuel containing hydrogen is passed over the anode and oxygen (air) is passed over the cathode. The reactants, through the electrolyte (membrane), react indirectly with each other generating an electrical voltage between the cathode and anode. Typical electrical potentials of PEM cells can range from 0.5 to 0.9 volts; the higher the voltage the greater the electrochemical efficiency. However, at higher current densities, the cell voltage is lower and there is eventually a peak value in power density for a given set of operating conditions.
Multiple cells are combined by stacking, interconnecting individual cells in electrical series. The voltage generated by the cell stack is effectively the sum of the individual cell voltages. There are designs that use multiple cells in parallel or in a combination series, parallel configuration. Separator plates (bipolar plates) are inserted between the cells to separate the anode reactant of one cell from the cathode reactant of the next cell. To provide hydrogen to the anode and oxygen to the cathode without mixing, a system of fluid distribution and seals is required.
A number of applications require a wide range of power; for example, the Unmanned Aerial Vehicle (UAV). High powers are required during take-off and climb, with lower average powers for cruise. A typical hybrid fuel cell and battery system would require a DC/DC converter to manage the different voltages from the battery and fuel cell. This is a disadvantage for system weight, volume and efficiency.
With a hybrid fuel cell/battery system, it is also a significant advantage to charge the batteries while in operation when there is available energy from the fuel cell. For this to occur in a safe and reliable manner, the maximum battery charging voltage and current must be carefully controlled. There are several methods to achieve this, but each has its own disadvantages.
For battery charging, one method is to use a buck DC/DC converter which will reduce the stack voltage down to the maximum allowable battery charge voltage. The issue with this approach is that there are losses associated (˜90% efficiency), and therefore the stack voltage must be significantly higher than the battery charge voltage to enable battery charging. This means that the battery will be charged only when the stack voltage is very high.
Another method is to use a buck-boost DC/DC converter which will supply the charge voltage at any stack input voltage. The issue with this approach is that when the electrical load requires high power levels, energy from the stack will be supplied directly to the bus through the battery charger, thereby imposing an unnecessary efficiency loss.
Thus, there is a need for improved power management and battery charging for a fuel cell/battery system.